Japanese Patent Application Publication No. JP-A-H05-288020 discloses a hydraulic lash adjuster used in valve gears. The lash adjuster comprises a bottomed cylindrical body and a bottomed cylindrical plunger which is accommodated in the body so as to be movable upward and downward. The plunger has an upper end which protrudes out of the body and is adapted to support a rocker arm.
The plunger has an interior serving as a low pressure chamber, and the body includes a lower interior that is partitioned by a bottom wall of the plunger thereby to serve as a high pressure chamber. The bottom wall of the plunger has an opening serving as a valve hole of a check valve. A hydraulic fluid is supplied into the low pressure chamber, and part of the hydraulic fluid in the low pressure chamber is caused to flow through the valve hole into the high pressure chamber, filling the high pressure chamber. The high pressure chamber also accommodates a valve element constituting a check valve and a valve spring biasing the valve element to the valve hole side (a valve-closed direction). When the plunger stands still, the bias of the valve spring holds the valve element in a valve-closed position where the valve hole is closed by the valve element, whereupon the check valve is closed.
When the plunger is moved upward, the vale element is moved downward against the bias of the valve spring, whereupon the check valve is opened. As a result, the hydraulic fluid in the low pressure chamber flows into the high pressure chamber. On the other hand, when the plunger is moved downward, the operating fluid in the high pressure chamber flows through a slight gap between an inner circumferential wall of the body and an outer circumferential wall of the plunger while the check valve is retained in the valve-closed state. As a result, the operating fluid flowing out of the high pressure chamber is returned through a communication hole extending through the circumferential wall of the plunger into the low pressure chamber.
When the rotating speed of the valve gear is increased in the above-described lash adjuster, a moving speed of the valve element is increased. Accordingly, an inertia force moves the valve element far away from the valve hole when the plunger is moved upward, whereupon valve element's arrival at the valve-closed position is retarded in the downward movement of the plunger. This results in concern that an amount of downward movement of the plunger becomes excessively large.
Increasing a biasing force of the valve spring is considered as a means for enhancing return of the valve element to the valve-closed position. In this case, however, there is a possibility that the valve element is kept closing the valve hole or that an opening degree of the valve hole becomes insufficient such that a smooth upward movement of the plunger is adversely affected.